Hitherto, optical memories have been developed mainly in two-dimensional recording-type optical discs such as a CD or a DVD, and a Blu-ray Disc. However, two-dimensional recording-type optical memories have already reached a diffraction limit, and thus a further increase in capacity is not likely to be achieved. Consequently, in recent years, three-dimensional recording-type optical memories have been developed actively. When a three-dimensional recording type is adopted, there is the possibility of recording capacity being increased to 100 to 1,000 or more than that in a two-dimensional recording type. 100 TB-level optical disc memories can also be realized theoretically.
Three techniques leading to an increase in an optical memory include 1) a near-field optical recording system, 2) a two-photon absorption memory, and 3) a holographic memory. 1) The near-field optical recording system is a recording system in which “near-field light” having light wavelength size or less is used. The near-field optical recording system is basically a two-dimensional recording-type technique, but has the likelihood of realizing high density recording exceeding a diffraction limit by using near-field light. In addition, 2) the two-photon absorption memory is a three-dimensional recording-type optical memory capable of three-dimensional access to a recording medium by using the intensity dependence of a nonlinear effect. Comparing with these techniques, 3) the holographic memory is an optical memory capable of three-dimensional recording without multi-layering a recording medium by performing multiplex recording on a hologram produced by interference between signal light and reference light.
All the optical memories of the above 1) to 3) achieve a recording capacity of approximately 500 GB to 1 TB at this moment in time. Therefore, from the viewpoint of recording capacity, there is no large difference in superiority or inferiority between the optical memories of the above 1) to 3). However, from the viewpoint of a data transfer rate, there is a large advantage in the holographic memory having a spatially two-dimensional massively parallel type input-output function out of the optical memories of the above 1) to 3). In recent years, a spatial light modulator (hereinafter, occasionally abbreviated to a “SLM”) for high-speed response exceeding a microsecond, or the like has also been developed. There is the possibility of a transfer rate exceeding 100 Gbps being realized by applying such a SLM for high-speed response to the holographic memory.
The holographic memory can realize both high density recording and a high data transfer rate, and thus is expected to be put into practical use as a next-generation optical memory. The recording capacity of the holographic memory being presently developed is approximately 600 GB to 1 TB/disc (see, for example, Non-Patent Literature 1). Since the recording capacity of one side of one platter of a HDD (having a size of 3.5 inches and a storage capacity of 2 TB) is 333 GB, the holographic memory is about 2 to 3 times superior to a magnetic recording medium which is put into practical use from the viewpoint of the recording capacity. In addition, in the holographic memory, the recording capacity thereof is considered to be further increased up to 10 to 100 times theoretically. Under such circumstances, with the purpose of increasing the recording capacity of the holographic memory, not only intensity modulation-type holographic memories having been used so far, but also phase modulation-type ones have been examined. However, there has been a problem in the phase modulation-type holographic memory in that since a phase modulation signal cannot be directly detected in a light detector, the phase modulation signal has to be converted into an intensity signal by some kind of method and then be detected.
The intensity modulation type is the most common modulation method, and many cases have been reported so far (see, for example, Non-Patent Literatures 1 to 3). In many recording systems using holography from the document (Non-Patent Literature 2) in which it was first suggested that information can be recorded using holography to the latest documents (Non-Patent Literatures 1 and 3) with a view to commercialization of products, two-valued (0 and 1) intensity modulation is used. However, there is a problem in that, the intensity modulation has an advantage of being able to construct a system through a simple optical system, whereas the exposure intensity difference between a central portion and a peripheral portion of a laser light irradiation region becomes large, and the dynamic range of a recording medium is consumed to a large extent, thereby resulting in the deterioration of recording efficiency. This problem is caused by the increasing exposure intensity difference between the central vicinity and the peripheral portion of the laser light irradiation region because the intensity in the vicinity of the center of a Fourier transform image is proportional to the sum of the amplitudes of all pixels in a general Fourier transform hologram (see, for example, Non-Patent Literature 4).
A method of alleviating such a problem of the intensity modulation type includes a method of using a modulation code in which two-valued information is dispersed into a plurality of pixels called a block and is coded, and data is represented by activating only a portion of the pixels within the block. Such a modulation code is used, thereby allowing an error due to crosstalk between pixels to be reduced. In addition, by using the modulation code, the exposure intensity difference between the central vicinity and the peripheral portion of the laser light irradiation region is reduced and the number of multiplex recordings is increased, thereby allowing effective recording to be performed (see, for example, Non-Patent Literatures 5 and 6). However, when the modulation code is used, the code rate defined by “(the number of bits recorded per block)/(the number of pixels per block)” falls short of 1. This means that the recording capacity per block in a case where the modulation code is used, in principle, falls short of the recording capacity in a case where the modulation code is not used.
In order to increase the recording capacity of the holographic memory, a method is required in which a plurality of pieces of information are recorded per pixel, that is, the code rate exceeds 1. In order to realize the code rate exceeding 1, it is required to use a multi-valued signal exceeding two values of 0 and 1. The multi-valued signal can be realized by dividing the light intensity into several levels, thereby allowing the code rate to be improved dramatically. However, in a current direct detection system, the signal-to-noise ratio of a reproduction light beam is greatly deteriorated with an increase in the multi-valued number due to the accuracy or noise of the detection system (see, for example, Non-Patent Literature 7).
In the intensity modulation system, the problem that the exposure intensity difference between the central portion and the peripheral portion of the laser light irradiation region becomes large and the dynamic range of a recording medium is consumed to a large extent can be solved even by a phase modulation system. The phase modulation system is a system that performs modulation using the phase of a light wave, and has recently attracted attention. For example, in the phase modulation system, when the phase of a light wave in a certain pixel is set to 0, information is represented by setting the phase of a light wave in another pixel to π. The numbers of pixels of 0 and π among the pixels included in two-dimensional page data produced by the spatial light modulator (SLM) are the same as each other, the exposure intensity difference between the central vicinity and the peripheral portion of the laser light irradiation region does not occur, and thus the useless consumption of the dynamic range of a recording medium can be suppressed. This point contributes significantly to an increase in the number of multiplex recordings. However, since a photoelectric conversion device such as a CCD is sensitive only to the intensity of light, the device cannot detect phase information directly. Therefore, in order to detect the phase information, the phase has to be converted into the intensity before light detection is performed. In the phase modulation system, this point becomes a serious problem.
Several phase detection methods for realizing a phase modulation-type holographic memory have been proposed so far (see, for example, Non-Patent Literatures 4, 8, and 9).
In Non-Patent Literature 4, an edge-detection method is proposed as a phase detection method used in a holographic memory. The edge-detection method is a method in which the feature of the phase modulation-type holographic memory is used well. In the phase modulation-type holographic memory, since the central intensity (direct-current component) of a Fourier transform image is missing, reproduction is performed only on other alternating-current components. This means that the intensity of a boundary portion of pixels of 0 and π in a reproduced image (real-space distribution) is emphasized. In other words, it means that the phases of all the pixels following the boundary of which the intensity is emphasized can be determined on the basis of a certain known pixel. There is a problem in that, this method has an advantage of being able to realize a phase modulation-type holographic memory in an optical system which is no different from an intensity modulation-type holographic memory, whereas the method is not adequate to the detection of a multi-valued phase modulation signal.
In Non-Patent Literature 8, a phase detection method using a birefringent medium is proposed. In this method, a reproduction light beam is changed to a circularly polarized light beam using a π/4 wavelength plate, and then is caused to pass through a birefringent medium. Reproduction light beams slightly shifted thereby interfere with each other, and an intensity pattern is obtained. Since the number of pixels shifted by a birefringent medium designed in advance can be determined, a determination can be made with the discarding of phase information from the obtained intensity pattern. This method is known to have a high misregistration resistance through an experiment in the document, and is a very attractive method in phase detection sensitive to a shift. However, even in this method, there is a problem in that a multi-valued signal is not easily detected, and that the higher-accuracy design of the birefringent medium is required.
In Non-Patent Literature 9, a beam phase lock-type collinear hologram is proposed as a phase modulation-type holographic memory specializing in a collinear optical system attracting attention as a one-beam recording system. This system is a system in which when a collinear hologram is reproduced, recorded holograms are simultaneously irradiated with phase-lock light of which the phase is known, in addition to normal collinear reference light, to thereby read recorded phase information as intensity information. In this system, since the phase-lock light passes through a recording hologram, the phase distribution thereof is influenced by propagation within the hologram having a phase diffraction grating. This can cause the occurrence of a phase error in a detection surface. This method is also not likely to record and reproduce the phase information with a high degree of accuracy, and the phase multi-valued number remains in two-valued to four-valued number.